A mother returning to her luxury New York apartment with her 3-year-old found her other two children stabbed to death in a bathtub and the kids' nanny lying unconscious nearby with apparently self-inflicted stab wounds to her neck, New York Police Commissioner Ray Kelly told reporters.

The mother and daughter were returning from a swimming lesson to the luxury La Rochelle apartment building on the city's affluent Upper West Side at approximately 5:30 p.m., Kelly said. When they found the apartment dark, they asked the doorman if the nanny had left with the other children only to be told that was not the case.

"She goes looking for her children and finds her 6-year-old daughter and 2-year-old son stabbed to death in the tub," Kelly said.

Other police officials said the younger child was 1 year old.

The mother's screams could be heard emanating from the apartment, Kelly said.

No charges were yet filed as the investigation proceeded "aggressively," Kelly said, but the "operating assumption" was that the nanny's wounds were self-inflicted.

A neighbor who lives on the same floor in the building at 57 West 75th Street near Columbus Avenue told The Associated Press that she heard screams around 5:30 p.m.

"There was some kind of screaming about, 'You slit her throat!' It was horrible," said the neighbor, Rima Starr, a music therapist.

Starr told AP she believed the nanny had been hired just recently.

"I met her in the elevator, the day before yesterday, and was making small talk," she said.

After police arrived, she told AP, the mother remained in the building's lobby, screaming hysterically and clutching her surviving child.

The nanny, approximately 50 years old, was in critical but stable condition and taken to an area hospital.

The children found in the bathtub were transported to St. Luke's Hospital, police said.

According to Kelly, the preliminary police investigation had not yet yielded a possible motive for the deaths.

Officials were not yet identifying the victims or the nanny, Kelly said.